1. Field of the Invention
The present invention relates to a multi-layer expansion-molded article of a polypropylene resin, which has a polypropylene resin layer on the surface of a foamed polypropylene resin layer, and is used as, for example, a light-weight heat insulating panel utilized for a floor, door or the like, a pallet, a container, an automotive member, or the like.
2. Description of the Background Art
Multi-layer resinous expansion-molded articles in various forms, in which a thermoplastic resin layer is formed on the surface of a foamed resin layer, have heretofore been known. In recent years, the problem of refuse has been causing great concern, and so there has been a demand for changing containers for fishery products made of an expansion-molded article of polystyrene resin particles, and the like, which have heretofore been disposable, to containers made of a multi-layer resinous expansion-molded article, in which a polypropylene resin layer is formed on the surface of a foamed polypropylene resin layer, to circulate them as returnable containers capable of sterilizing with steam. Automotive members and the like have also been required to constitute with members made of the same multi-layer resinous expansion-molded article having a polypropylene resin layer on its surface as described above from the viewpoints of recycling ability, strength, lightweight property, heat insulating property and sound absorbing property.
As this kind of multi-layer resinous expansion-molded articles, there have been known those obtained by a process in which a molded article of a hollow structure is formed with a thermoplastic resin to use it as a resin layer, and a polyurethane resin is poured into the cavity within the molded article, or foamed beads of a resin are filled thereinto to mold (Japanese Patent Publication No. 10217/1983, Japanese Patent Application Laid-Open No. 339979/1994, etc.). However, this process has involved problems that the molding process of the multi-layer resinous expansion-molded article becomes complicated, and a special molding machine is required, since a step of forming the resin layer which becomes a skin and a step of forming a foamed layer in the inside of the resin layer are separately conducted. It has hence been difficult to provide the multi-layer resinous expansion-molded article on the cheap.
There has also been proposed a process in which a hollow multi-layer parison with a thermoplastic resin layer provided on the surface of a foamed resin layer is molded while compressing it in a mold to cause the opposed inner surfaces of the parison to be fusion-bonded to each other, thereby obtaining a multi-layer resinous expansion-molded article (Japanese Patent Publication No. 27978/1987, Japanese Patent Application Laid-Open No. 312449/1994, etc.).
However, such a process of molding the multi-layer parison while compressing it in the mold as described above has involved a problem that it is difficult to provide any expansion-molded article having a resin layer having a uniform thickness, and so irregularities occur in the surface of the resulting expansion-molded article to impair its appearance, or the resin layer cracks or bores when a molded article having a relatively high draw ratio, such as a container, is formed. In addition, this process has also involved a problem that when a polypropylene resin is used as a base resin, it is difficult to obtain a molded article having a low-density foamed resin layer the density of which is as low as at most 0.4 g/cm3, or when it is attempted to make the density of the foamed resin layer low, the resulting foamed resin layer tends to cause cell collapse and formation of open cells. As described above, this process has involved some problems to be solved.